Computerized tomography scanners are medical diagnostic instruments in which the subject is exposed to a relatively planar beam or beams of x-ray radiation, the intensity of which varies in direct relationship to the energy absorption along a plurality of subject body paths. By measuring the x-ray intensity (i.e., the x-ray absorption) along these paths from a plurality of different angles or views, an x-ray absorption coefficient can be computed for various areas in any plane of the body through which the radiation passes. These areas typically comprise approximately a square portion of about 1 mm.times.1 mm. The absorption coefficients are used to produce a display of, for example, the bodily organs or structural members of industrial equipment intersected by the x-ray beam.
An integral and important part of the scanner is the x-ray detector which receives the x-ray radiation which has been modulated by passage through the particular body under study. Generally, the x-ray detector contains a scintillator material which, when excited by the impinging x-ray radiation, emits optical wavelength radiation. In typical medical or industrial applications, the optical output from the scintillator material is made to impinge upon photoelectrically responsive materials in order to produce electrical output signals, the amplitude of which is directly related to the intensity of the impinging x-ray radiation. The electrical signals are digitized for processing by digital computer means which generates the absorption coefficients in a form suitable for display on a cathode ray tube screen or other permanent media.
Due to the specific and demanding computerized tomography requirements, not all scintillator materials which emit optical radiation upon excitation by x-ray or gamma ray radiation are suitable for computerized tomography applications. Useful scintillators must be efficient converters of x-ray radiation into optical radiation in those regions of the electromagnetic spectrum (visible and near visible) which are most efficiently detected by photosensors such as photomultipliers or photodiodes. It is also desirable that the scintillator have a high optical clarity, i.e., transmit the optical radiation efficiently to avoid optical trapping, such that optical radiation originating deep in the scintillator body escapes for detection by externally situated photodetectors. This is particularly important in medical diagnostic applications where it is desirable that x-ray dosage be as small as possible to minimize patient exposure, while maintaining adequate quantum detection efficiency and a high signal-to-noise ratio.
Among other desirable scintillator material properties are short afterglow or persistence, low hysteresis, high x-ray stopping power, and spectral linearity. Afterglow is the tendency of the scintillator to continue emitting optical radiation for a time after termination of x-ray excitation, resulting in blurring, with time, of the information-bearing signal. Short afterglow is highly desirable in applications requiring rapid sequential scanning such as, for example, in imaging moving bodily organs. Hysteresis is the scintillator material property whereby the optical output varies for identical x-ray excitation based on the radiation history of the scintillator. Hysteresis is undesirable due to the requirement in computerized tomography for repeated precise measurements of optical output from each scintillator cell and where the optical output must be substantially identical for identical x-ray radiation exposure impinging on the scintillator body. Typical detecting accuracies are on the order of one part in one thousand for a number of successive measurements taken at relatively high rate. High x-ray stopping power is desirable for efficient x-ray detection. X-rays not absorbed by the scintillator escape detection. Spectral linearity is another important scintillator material property because x-rays impinging thereon have different frequencies. Scintillator response must be substantially uniform at all x-ray frequencies.
Compositions and methods of forming polycrystalline, rare earth oxide, ceramic scintillators having high optical clarity, density, uniformity, cubic structure, and useful in computerized tomography scanners, are disclosed in U.S. Pat. Nos. 4,421,671, 4,518,545, 4,525,628, 4,466,929, 4,466,930, and 4,747,973, incorporated herein by reference. Briefly described, the polycrystalline ceramic scintillators are formed of a rare earth oxide selected from the group consisting of Gd.sub.2 O.sub.3, Y.sub.2 O.sub.3, La.sub.2 O.sub.3, Lu.sub.2 O.sub.3, and mixtures thereof that form a cubic crystal structure. The rare earth oxide is doped with a rare earth activator such as europium, neodymium, ytterbium, dysprosium, terbium, and praseodymium to form the cubic crystal structure which scintillates at a predetermined wave length. Optionally, transparency promoters such as ThO.sub.2, ZrO.sub.2, and Ta.sub.2 O.sub.5 can be added in an amount sufficient to improve the transparency of the ceramic scintillator, and a light output restorer such as CaO or SrO in an amount sufficient to effect a higher light output.
An important step in forming the rare earth oxide ceramic scintillators is the preparation of a suitable powder containing the desired scintillator material constituents. Suitable powders have submicron-to-micron powder particles, and are, for example, 99.99% to 99.9999% pure. Powder particle size is submicron-to-micron to provide high optical clarity, larger particle size results in higher porosity and loss of optical clarity when the powder is sintered to form the scintillator body. A known method for forming the desired starting powder employs a wet chemical oxalate precipitation process. The selected molar percentages of the nitrates of yttrium, gadolinium, europium, niobium, ytterbium, dysprosium, terbium, and praseodymium, are dissolved in water. The aqueous nitrate solution of the desired scintillator material constituents is admixed with a solution of oxalic acid which is, for example, 80% saturated at room temperature. The resulting coprecipitated oxalates are washed, neutralized, filtered, and dryed in air at about 100.degree. C. for approximately 8 hours. The oxalates are then calcined, thermally decomposed, in air at approximately 700.degree. C. to about 900.degree. C. for a time ranging from one to four hours to form the corresponding oxides. Typically, heating for one hour at 800.degree. C. is sufficient. Preferably, the oxalates or the resulting oxides are milled by one of several methods such as ball, colloid, or fluid energy milling to enhance optical clarity when the powder is sintered to form the scintillator.
Selected amounts of the powder composition are formed into powder compacts by either die pressing, or die pressing followed by isostatic pressing to further increase green density. The compact is densified by sintering, sintering plus gas hot isostatic pressing, or ceramic hot pressing methods. In the known methods for forming the rare earth ceramic scintillator materials described above, optical clarity is most improved in the sintered scintillators formed from milled oxalate or oxide powders.
It is an object of this invention to provide a simplified method for forming high optical clarity yttria-gadolinia ceramic scintillators without performing the step of milling the oxalate or oxide powders.
It is another object of this invention to form finely divided yttria-gadolinia ceramic scintillator powders by a wet chemical method where hydroxide coprecipitates are formed, followed by conversion of the hydroxides to oxalates.
The terms "transparency" and "translucency", as used herein, describe various degrees of optical clarity in the scintillator material. Typically, the inventive scintillator materials exhibit an optical attenuation coefficient of less than 100 cm.sup.-1, as measured by standard spectral transmittance tests (i.e., "narrow" angle transmission) on a polished scintillator material plate, at the luminescent wavelength of the respective ion. The most desirable scintillator materials have lower attenuation coefficients and hence higher optical clarity, i.e., higher transparency.